for OS authentication for SYSDBA to work on Windows OS, your Windows (domain) user must be a member in the local ORA_DBA group
(direct, and not via another group) and you must have the sqlnet.ora parameter :

sqlnet.authentication_services = (NTS)

configured. An alternative is to use a passwordfile.

For More information see: Note 77665.1 WIN: OS Authentication - CONNECT AS SYSDBA Without a Password